The invention relates to a safety device for use on compressing, shearing and closing edges in power-operated apparatus such as door and gate systems, presses and the like, with a signaller arranged substantially along the entire edge and connected to the drive of the power-operated apparatus via a control unit.
In known safety devices of this kind, the signaller is a flexible tube filled with air and connected to the control unit for the drive via a pressure wave switch. If the tube is compressed, for example by a person being caught between the closing door or doors of a lift, a pressure wave is produced which is converted by an electromechanical member and causes the drive to stop or operate in the reverse direction. A safety device of this kind has the disadvantage that it is not self-monitoring. If, for example, the air-filled tube has a defective or flawed area or is leaky, it is possible that the pressure wave produced on compression of the rubber hose will not reach the electromechanical transducer, since compressed air is escaping outwards at the defective point. However, this means that the safety device is out of order and will fail, without anyone having previously noticed the fact. This is a danger chiefly in power-operated devices, such as door and gate systems, mounted in the open air and thus exposed to the effects of weathering, with the result that, after a lengthy period of operation, the air tube used will in all probability cease to be sound.
Self-monitoring safety devices are known, such as so-called light barriers and ultrasonic barriers, for example. However, these known devices are very expensive, for one thing, and this in itself restricts their possible applications. What is more, it is not possible with these devices to secure edges uniformly along their entire length, e.g. along the main closing edge of a sliding door, since these devices can only be used at individual points.
A pressure-sensitive switch is also known, which contains a preferably cushion-shaped member made from a synthetic semiorganic elastic non-conductive substance such as silicone rubber, in which a quantity of individual, electrically conductive metal particles are dispersed. The dispersion of the particles is such that when the cushion is in its normal state, i.e. not under pressure, the electrical resistance of the cushion is infinitely great and thus the cushion is non-conductive. If the cushion is subjected to sufficiently great compressive forces, the particles are correspondingly pressed together into mutual engagement and the resistance of the cushion changes so that the cushion becomes electrically conductive. When the compressive force is released, the cushion returns to its normal uncompressed state, by its inherent elasticity, and the particles in turn move relative to one another and the cushion becomes non-conductive. This switch is not self-monitoring either, since no signal is produced if the cushion is damaged or torn away, for example.
The object of this invention is to provide a safety device of the kind described hereinbefore which is self-monitoring but can nevertheless be used economically to secure compressing, shearing and closing edges uniformly over their entire length. The safety device should, in particular, be useable in all apparatus in which dangerous shutting movements occur and should also safely prevent people or objects from being trapped in the case of breakdown.